W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Beam-beam: Experimental Status  Introduction: PEP-II collision parameters & recent performance  Interplay between e - - cloud & beam-beam issues  Beam-beam limit studies  Summary W. Kozanecki, CEA-Saclay

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 IP Parameter Design Peak performance (Jun 03) C-M energy (GeV) (e + : 3.1 ; e - : 9.0) Crossing angle (mrad) 0.0< 1.0 Luminosity (x /cm 2 /s) Number of bunches LER current (mA, e + ) HER current (mA, e - ) LER/HER current ratio 2.9/11.3/1  y */  x * (cm/cm) 1.5 / / 40+, 1.2 / 28- Emittance (nm-rad) (y/x) 1.5 / / 30+, 1.8 / 49- IP rms beam size  y /  x (  m) 4.7 / / 113 LER tunes (x/y) / / HER tunes (x/y) / / Beam-beam parameter (vertical +/-) / Beam-beam parameter (horizontal +/-) / PEP-II Collision Parameters J. Seeman, Jun 03

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Typical recent performance (May-Jun 03) Luminosity (  b -1 ) I + x I - (mA -2 ) Specific luminosity (  b -1 mA -2 b -2 ) I + x I - (mA -2 )  Near ½ integer ( + x/y ~ 0.52/0.57)    e - y-size  with  e + current    e + x-size  with  e - current  Total luminosity: some tune-shift saturation, but potential for more L  Specific luminosity scales (primarily) with e + current

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Interplay between e - - cloud & beam-beam issues Bunch-by-bunch luminosity versus position along the whole train. Pattern: ‘by-4’ (8.4 ns spacing) with 7 additional big gaps, July The first bunches of each mini-train have a high luminosity, which drops to 40 % of its initial value at the end of the longest train. The long gaps clear the electron cloud, which slowly builds up again over along the mini- train. Solenoids had been installed in part of the straights only. Standard luminosity pattern in spring 2003 Pattern: ‘by-3’ (6.3 ns spacing) Mini-trains of 10 and 11 bunches are alternating. There is an ion gap of about 3%. In this pattern each mini-train has constant luminosity (except for bunches 1+3). Solenoids now cover most of the beam pipe in all straights and arcs. F.-J. Decker, et. al., PAC’01, ‘03

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Interplay between e - - cloud & beam-beam issues: impact  Bunch pattern optimization: maximize I bunch, taking into account  total-current budget (RF power, beam-heating problems)  minitrain spacing (larger minigaps => better e - cloud suppression)  minitrain length (shorter minitrains => less e - cloud buildup)  # of minitrains (fewer minitrains => fewer ‘fragile’ bunches)  need for current ramps at start of train (and/or minitrains)  The severity of electron-cloud effects (for a fixed bunch pattern) has been steadily decreasing over the years  Low-field (25-35 G) solenoids now cover most of the accessible beam- pipe sections. This system was upgraded this summer ( up to 3x higher field in the straights + cover remaining short sections)  Vacuum-pipe scrubbing appears to have played a significant role  Some e – cloud effects are no longer apparent  single-beam e + blowup at high I + no longer observed (but what once I +  ?)  In typical recent running, only 1st (few) bucket(s) in each minitrain affected by electron cloud (if any)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Interplay between e - - cloud & beam-beam issues: towards higher luminosities  Impact of e - cloud may be more severe once higher currents force the use of a denser pattern (‘by-2’) e - cloud Pattern: ‘by-2’ (4.2 ns spacing), long trains (Feb 03) 20-25% luminosity loss after the first 5-10 bunches in each train Pattern: ‘by-2’ (4.2 ns spacing), short trains [ ] (Apr 03) The first buckets of the duplets have the same high L, while the L of the second ones drops about 50% over 320 ns. It remains constant for several hundred ns, then slowly grows to nearly 75% before it starts dropping again. F.-J. Decker, et. al.,PAC’ 03

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Beam-beam limit studies  Experimental procedure  Fix one beam current (typically similar to physics conditions)  Vary the current of the other beam from 0 to maximum possible; at each setting, optimize luminosity on tunes  Measure L/bunch, specific luminosity L sp, individual beam sizes  - x,y in- & out-of-collision  Diagnostics  Fast luminosity monitor (e + e -  e + e -  )  Horizontal beam sizes: synchrotron-light monitor (SLM)  Vertical beam sizes: SR-light interferometer

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 HER b-b high e - current: I - =625 mA, I + = mA, 597 bunches L SP drops by ~ 25-30% W.K., PEP-II Performance Workshop, Dec 00 Collisions: LEB blowup in both x & y (N.B.: y blowup instrumental?) Single-beam: LEB blowup minimal LEB blowup in collision >> single-beam blowup, by an amount that depends on its own current => L SP drops! HER:SLM x & y size in collision flat with I + No indication of b-b induced HER blowup (in this data set)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 LER b-b high e + current: I + =1070 mA, I - = mA, 597 bunches LER:SLM x & y size in collision >> single-beam, flat with electron current (not “naive” b-b!) HER: I - dep. of x, y SLM size in collision, consistent with single-beam behavior (flat) L SP drops by < 5% LER b-b low e + current: I + = 90 mA, I - = mA, 829 bunches

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 6 Feb 03 data Typical current-dependence of specific luminosity & beam sizes, Jan-Apr 03

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Wandering in tune space...  LER   Apr 03:  x =.64  y =.56  since May 1, 2003:  x =.52  y =.57  HER   Apr 03:  x =.57  y =.64  since May 1, 2003:  x =.52  y =.62 The values quoted here are nominal, unshifted tunes

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 “Before” ½ integer vs. “after”: LER beam sizes LER y-size (28 Apr) +7 % LER x-size (28 Apr) +11 % LER current (e + ) [mA] % wrt  x ( single-beam) + 8 % LER x-size (2 May) HER current (e + ) [mA] LER y-size (2 May) + 12 % HER current (e + ) [mA]

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 “Before” ½ integer vs. “after”: HER beam sizes & Luminosity HER y-size (2 May) + 39 % HER y-size (28 Apr) L/bunch (28 Apr) L/bunch (2 May) LER current (e + ) [mA] Product of bunch currents [(mA2/b) 2 ]

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 L SP (2 May 03) Msrd: - 25 % Predctd: -25% Spec. luminosity [10 30 cm -2 s -1 mA -2 /b] LER current (e + ) [mA]  Original tunes ( + x/y ~ 0.64/0.56)  e - size ~ independent of e + current    e + size ~  with  e + current (mostly x: proximity to 2/3?)  Near ½ integer ( + x/y ~ 0.52/0.57)    e - size  with  e + current    e + size  with  e - current  Specific luminosity  scales (primarily) with e + current  => HEB the ‘weaker’ beam (at that time...)  Total luminosity  some tune-shift saturation (prob. HEB), but potential for more luminosity! I bunch (e - ) x I bunch (e + ) [(mA/b) 2 ] L / bunch (10 30 cm - 2 s -1 b -1 ) Luminosity/bunch (2 May 03)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Dynamic-  effect? (skipped these runs) Measurement of luminous-region size by BaBar (dimuons & Bhabha’s) 1/  L = sqrt (1/  /  - 2 )  Reduction of horizontal spot size from ~ 100  m to ~ 70  m.  No changes observed in longitudinal spot size. before April 28thafter April 28th x ~ 0.5 M. Baak, Sep 03

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Tune spectra in collision + x + x + y + y - x - x - y - y Old tunes x ~ 0.5 (June 03) ?

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 LER y-tune: b-b tune shift!   ~.004 for 0.81 < I - < 1.07 A HER y-tune: not fully compensated HER bunch current (e - ) [mA/b] LEB x-size + 64 % wrt  x ( single-beam) HER bunch current (e - ) [mA/b] y-tunes in collision, vs. e - bunch current (LEB trickle, 12 Jun 03)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Determination of beam-beam parameters from luminosity & spot-size data

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 “Measured” beam-beam parameters

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Beam-beam performance summary J. Seeman, Aug 03

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Summary (I)  Electron-cloud effects have been minimized by a combination of scrubbing, solenoid-suppression, and bunch-pattern optimization  Luminosity (& background!) optimization relies on a delicate balance between the currents, tunes, beam-beam parameters and e-cloud effects as these parameters vary along each bunch train.  Current-dependence of the spot sizes in collision x ~ 0.5)  LEB (HEB) sizes depend only on e - (e + ) current   y (e - ) grows by 30-70% wrt the e - single-beam size  30-40% is more typical of recent (stable) running : mostly ‘spontaneous’ ?   x (e + ) grows by a factors of 1.5 to > 2 wrt the e + single-beam size  50-60% is more typical of recent (stable) running: ‘spontaneous’ or effective optimum?  offline analysis of luminous region size (using dimuons & Bhabh’as)  corroborates the expected reduction in IP x-spot size (  simulation?) ...but the observed variations of IP spot size are not always consistent with the measured current-dependence of individual e + and e - beam sizes

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Summary (II)  Beam-beam parameter measurements  some tune measurements in collision are difficult (esp. LER x )  extracting  (or limits thereon) from L and  (SL  +,- x,y promising, but...  it needs better control of SLM/interferometer systematics  it would greatly benefit from the ability to ‘translate’ SL sizes into IP sizes  the interpretation/analysis is complicated by dynamic-  effects.  1 measurement of vertical LEB beam-beam tune-shift vs. HER current! but interpretation not straightforward..  can we gain more from such parasitic monitoring?  New diagnostics being commissioned  gated cameras in HER & LER  on-line measurements of IP positions & spot sizes by BaBar  gated tune monitor  PEP-II has recently achieved, near the ½ integer, beam-beam parameters  x /  y of about.109 /.082 (.040 /.040) in the LER (HER)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Spare slides

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03  In contrast to ‘classical’ single-ring collider,   + x,y   - x,y (and  + x,y   - x,y only) =>  + x,y   - x,y  interpreting luminosity in terms of  requires additional knowledge and/or assumptions on individual IP spot sizes  energy-transparency conditions  + x,y =  - x,y I + b E + = I - b E - (provided  + x,y =  - x,y,  + x,y =  - x,y, + x,y = - x,y...)  largely violated in PEP-II  best performance repeatedly achieved with I + / I – ~ 1.3 (more typically 1.7 – 2, not 2.9!)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Typical performance since Run 4 startup I + x I - (mA -2 ) Luminosity (  b -1 ) I + x I - (mA -2 ) Specific luminosity (  b -1 mA -2 b -2 )

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03  At high I +, e - cloud strength varies along minitrain =>  e + beam size varies (long range + within train) => Luminosity varies  e - beam-beam tune-shift varies (=> e - beam size may vary ??)  tunes optimized on the average only =>  slight L loss  ‘raining’ buckets (rapid loss of charge, background spikes, flip-flop)  electron-cloud enhanced beam-beam blowup of the e + beam Interplay between e - - cloud & beam-beam issues (2) Gated-camera measurements (2001 data, 4-by-22 pattern) R. Holtzapple, PEP-II Performance Workshop, Jan 2002

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Beam-beam flip-flop   Near the top of a fill:  Several LER bunches have reduced beam size (~30% reduction in beam size)  Several bunches - typically near the head of the train - have ~ ½ L  2 states  2 states of low-luminosity bunches: they have either reduced LER current or reduced LER beam size  I b + = low  I b + = low; I b - = average  x, y + = smal  I b +, I b - = average;  x, y + = small. Small LER bunches + low L implies transverse blow-up of the e - beam, since bunch currents are not particularly low for these. R. Holtzapple, et. al., SLAC-PUB-9238 (2001 data)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Beam-beam flip-flop (2)   Single Bunch Transition to low-luminosity state  Initially at average beam size  Luminosity dropped when rings were filled  The bunch experiences a sudden change in luminosity/beam size  This bunch went from one unstable state (low luminosity / small x, y) to the other unstable state (short lifetime)  Transition between states is fast (~0.5 sec.)  Horizontal beam size oscillation accompanies the transition

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Beam-beam flip-flop (c’td)   Possible Explanation of Beam Size Flip- Flop Dynamics  The LER bunches at the front of the train have a smaller transverse beam size (lower electron cloud density). These small (strong) LER bunches blow-up the HER bunches.  The resulting tune shift for the these LER bunches is smaller than “normal”, and as a result, they have a horizontal tune located near a resonance which gives them a shorter lifetime.  The LER bunches lose charge. Eventually the HER becomes strong enough to flop itself, and the LER bunch, back to “normal” size.  To confirm this theory, a 2 nd gated camera has been installed in the HER and is being commissioned. Start of the store (high I): L/bunch (almost) uniform throughout each train. End of the store (low I): single-bunch L dropouts are prevalent in the 1 st few trains

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 HER b-b limit (continued) LEB blowup in collision >> single-beam blowup, by an amount that depends on its own current => L SP drops! LEB blowup with both beams present but out of collision, is similar to single-beam blowup No indication of b-b induced HER blowup (in this data set) Collisions: LEB blowup in both x & y (N.B.: y blowup instrumental??) Single-beam or separated: LEB blowup minimal By-5 - 1/50th, 597 bunches,  * y = 1.25, July 00

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 LER b-b low e + current: I + = 90 mA, I - = mA, 829 bunches LER: definite, but moderate, increase in both x,y SLM sizes with electron current (as expected) HER: I - dep. of x, y SLM size in collision, consistent with single-beam behavior L SP drops by ~ 10-15%

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 LER b-b e + current: I + =1070 mA, I - = mA, 597 bunches LER:SLM x & y size in collision >> separated, but both are flat with electron current (not “naive” b-b!) HER: I - dep. of x, y SLM size in collision, consistent with single-beam behavior (flat) L SP drops by < 5%

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 6 Feb 03 data Comparison of individual beam sizes with luminous-region measurements

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03  Very little HEB current-dependence of either LER or HER spot size  LEB current-dependence : for 1.1 < I + < 1.85 A,  LER x-size:+ 20% y- : %  HERx-size: + 6% (but: large spread) y- :+ 10% (but: large spread; may be consistent with ~0)  L sp : - 30% (measured) : - 21% (predicted from spot size variations) L sp L sp =2.9 predicted from HER/LER beam size relative variation 5 Apr 03 data LER bunch current (mA/bunch)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Specific luminosity vs. beam currents I - (mA) I+I+ L SP I - (mA) I + (mA) L SP Msrd: - 25 % Predctd: -25%

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 LEB size vs beam currents LER x-size + 8 % LER x-size LER y-size + 12 %

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 “Before” ½ integer vs. “after”: LER beam sizes LER x-size (28 Apr) LER y-size (28 Apr) + 8 % LER x-size (2 May) LER y-size (2 May) + 12 % +11 % +7 %

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 LER y-tune: b-b tune shift!   ~.004 for 0.81 < I - < 1.07 A LER y-tune knob HER y-tune: not fully compensated HER y-tune knob HER bunch current (e - ) [mA/b] y-tunes in collision, vs. e - bunch current (LEB trickle, 12 Jun 03)

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 No apparent dependence of the horizontal luminous size on LER or HER currents No apparent dependence of the longitudinal luminous size on LER current. Luminous spot sizes vs. beam currents Luminous spot sizes vs. beam currents after April 28 th x ~ 0.5 M. Baak, Sep 03

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Current-dependence of measured SL spot sizes

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Current-dependence of estimated IP spot sizes

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 Comparison of fitted & measured L & L SP

W. KozaneckiPEP-II MAC Review, 9-11 Oct 03 But it does not always work so well Jun3 May